As control-device power supplies such as general-purpose engines for driving various types of working machines, there are known power supplies which utilize the outputs of power generators which are driven by engines. This type of power supply devices are required to ensure sufficient electric power, even when the engine has a lower rotation speed and, thus, the power generator generates a lower input voltage. However, if such a power supply device is set to ensure sufficient electric power over a lower rotation-speed range, this will cause the problem of greater electric power losses at higher rotation speeds and, therefore, greater input voltages from the power generator.
To cope with the aforementioned problem, there has been suggested a system which supplies the output of a power generator, as a power supply, to a control device, through a transformer, as described in, for example, Japanese Patent Application Laid-open No. 2002-51591. This system can suppress the rise of the voltage with the transformer to provide required electric power. Patent Document 1: JP 2002-51591 A